Natural human hearing provides stereo perception whereby a listener can discriminate the direction from which a sound originates. This listening ability arises because the time of arrival of an acoustic signal at each respective ear of the listener depends on the angle of incidence of the acoustic signal. The amplitude of the acoustic signal at each respective ear of the listener can also depend on the angle of incidence of the acoustic signal. The difference between the time of arrival of the acoustic signal at each respective ear of the listener, and the amplitude of the acoustic signal at each respective ear of the listener, are examples of binaural cues which enrich the hearing perception of the listener and can enable certain tasks or effects. However, when acoustic sound is processed by a digital signal processing device and delivered to each respective ear of the user by a speaker, such binaural cues are often lost.
Processing signals from microphones in consumer electronic devices such as smartphones, hearing aids, headsets and the like presents a range of design problems. There are usually multiple microphones to consider, including one or more microphones on the body of the device and one or more external microphones such as headset or hands-free car kit microphones. In smartphones these microphones can be used not only to capture speech for phone calls, but also for recording voice notes. In the case of devices with a camera, one or more microphones may be used to enable recording of an audio track to accompany video captured by the camera. Increasingly, more than one microphone is being provided on the body of the device, for example to improve noise cancellation as is addressed in GB2484722 (Wolfson Microelectronics).
The device hardware associated with the microphones should provide for sufficient microphone inputs, preferably with individually adjustable gains, and flexible internal routing to cover all usage scenarios, which can be numerous in the case of a smartphone with an applications processor. Telephony functions should include a “side tone” so that the user can hear their own voice, and acoustic echo cancellation. Jack insertion detection should be provided to enable seamless switching between internal to external microphones when a headset or external microphone is plugged in or disconnected.
Wind noise detection and reduction is a particularly difficult problem in such devices. Wind noise is defined herein as a microphone signal generated from turbulence in an air stream flowing past microphone ports, as opposed to the sound of wind blowing past other objects such as the sound of rustling leaves as wind blows past a tree in the far field. Wind noise can be objectionable to the user and/or can mask other signals of interest. It is desirable that digital signal processing devices are configured to take steps to ameliorate the deleterious effects of wind noise upon signal quality. One such approach is described in International Patent Publication No. WO 2015/003220 by the present applicant, the content of which is incorporated herein by reference. This approach involves mixing the signals from at least two microphones so that the signal which is suffering from least wind noise is preferentially used for further processing. Such mixing is applied at low frequencies (e.g. less than 3-8 kHz), with higher frequencies being retained in separate channels. Other applications may require subband mixing at mid- and/or high frequencies in the audio range. However these and other methods of microphone signal mixing can corrupt the binaural cues being delivered to the listener.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
In this specification, a statement that an element may be “at least one of” a list of options is to be understood that the element may be any one of the listed options, or may be any combination of two or more of the listed options.